The present invention relates to a method and apparatus which processes a material by using a method and apparatus for determining an endpoint of a semiconductor device manufacturing process. More specifically, the present invention relates to a method and apparatus for determining an endpoint of semiconductor device manufacturing process and a method and apparatus for processing a material by using the endpoint determining method and apparatus, which detects an etch quantity of, or deposition on, a material being processed in a semiconductor integrated circuit manufacturing process. More specifically, the present invention relates to a method and apparatus for measuring an etch depth and a thickness of a material being processed and a method and apparatus for processing a material by using the etch depth/thickness measuring method and apparatus, which can precisely measure etch quantity of each of various layers formed over a substrate during an etching process using plasma discharge and thereby realize a desired etch depth and thickness of each layer.
In the manufacture of semiconductor wafers, dry etching has been in wide use in etching layers of various materials, particularly dielectrics, and in forming patterns over the wafer surface. What is most important in controlling process parameters is to accurately determine an etching endpoint during the processing of these layers at which desired etched depth and film thickness are reached and the etching is stopped.
During the dry etching of a semiconductor wafer, the intensity of light of a specific wavelength in a plasma beam changes as the etching of a particular film proceeds. An example method currently available for determining an endpoint of the semiconductor wafer etching process involves detecting a change in the intensity of light of a particular wavelength emitted from a plasma during the dry etching and, based on the result of detection, determining an etching process endpoint for a particular film. In this case, it is necessary to prevent an erroneous detection caused by noise-induced variations of a detected waveform. Methods for detecting light intensity variations with high precision includes those described in Japanese Patent Unexamined Publication Nos. 61-53728 and 63-200533. Noise reduction is achieved by a moving average method described in Japanese Patent Unexamined Publication No. 61-53728 and by approximation processing using a first-order least squares method described in Japanese Patent Unexamined Publication No. 63-200533.
As semiconductors are fabricated in increasingly microfine structures and in higher circuit densities in recent years, an open area ratio (an area of a semiconductor wafer to be etched with respect to its overall area) decreases, weakening the intensity of light of a specific wavelength from a reactive product that is taken into an photodetector from an optical sensor. As a result, a level of signal sampled from the photodetector becomes small, making it difficult for an endpoint determining unit to determine the etching process endpoint reliably based on the sampled signal from the photodetector.
In stopping the etching process after detecting the etching process endpoint, it is important that the remaining thickness of a dielectric layer be equal to a predetermined value. The conventional techniques monitor an overall process by using a time-thickness control method which assumes that the etch rate in each layer is constant. The etch rate is determined in advance as by processing a sample wafer. This approach uses a time monitor technique and stops the etching process when a length of time corresponding to a predetermined film thickness to be etched has elapsed.
An actual layer, for example, an SiO2 layer formed by the LPCVD (low pressure chemical vapor deposition) technique, however, is known to have a low reproducibility in terms of thickness. An allowable error of thickness due to LPCVD process variations is equal to about 10% of an initial thickness of the SiO2 layer. Hence, the time monitor technique cannot precisely measure the actual final thickness of the SiO2 layer remaining on a silicon substrate. The actual thickness of the remaining layer is measured at a final step by a technique using a standard spectroscopic interferometer. When it is decided that the wafer is overetched, the wafer is discarded as a faulty product.
It is also known that the insulating film etching apparatus has performance variations with elapse of time, such as the etch rate falling as the etching operation is repeated. In some cases the etching may inadvertently stop while in process. These problems need to be solved. In addition, it is important to monitor etch rate variations over time in assuring a stable execution of the process. The conventional method, however, monitors only the time in determining the etching process endpoint and does not provide an appropriate means of dealing with variations over time of the etch rate. When the duration of etching is as short as about 10 seconds, the endpoint determining method must reduce a decision preparation time and the decision interval be made short enough. These requirements are not met satisfactorily. Another problem is that an etched area of the insulating film is in many cases less than 1%, which means that a change in intensity of plasma-induced light emitted from a reactive product created during etching is small. Hence an endpoint determining system capable of detecting even very small changes is required, but no practical and inexpensive systems with such a capability are available.
Among other etching process endpoint determining methods for semiconductor wafers are those using an interferometer which are disclosed in Japanese Patent Unexamined Publication Nos. 5-179467, 8-274082, 2000-97648 and 2000-106356. These methods using an interferometer apply a monochromatic radiation from a laser at an orthogonal incidence angle to a wafer that includes a stacking structure of different materials. In a structure in which an SiO2 layer is stacked over an Si3N4 layer, for example, the radiated light reflected by the upper surface of the SiO2 layer and the radiated light reflected by a boundary surface formed between the SiO2 layer and the Si3N4 layer combine to form interference fringes. The reflected light is radiated onto an appropriate detector which generates a signal whose magnitude changes according to the thickness of the SiO2 layer being etched. During the etching process, as soon as the upper surface of the SiO2 layer is exposed, it becomes possible to monitor continuously and precisely the etch rate and the present thickness of the layer being etched. Another method is also known which uses a plasma, rather than a laser, to emit predetermined light which is measured by a spectrometer.